The present invention relates to a carrier of low electrical resistance for use in a two-component magnetic developer and a method of electrophotographically forming visual image using the carrier.
It is known that electrophotographic processes and electrostatic recording or printing methods are generally used to duplicate or reproduce an analog or digital data such as characters, graphics, etc. For instance, in an electrophotographic copying machine or facsimile, a photosensitive layer or dielectric layer is charged to a uniform potential and the charged portion is exposed to light image of original data to form a electrostatic latent image. The latent image is developed by bringing a magnetic brush into contact with the latent image, thereby adhering a toner to the latent image to form a visual toner image. Generally, the toners used there are those which are electrostatically chargeable to a polarity by a frictional contact with carrier particles, or magnetic toners which are composed primarily of a magnetic powder and a resin binder. However, such an electrophotographic image-forming method requires the electrophotographic copying apparatus to have an electrostatic latent image-forming means including charging unit, in addition to a developing means, to uniformly charge a surface of an image-bearing member, resulting in complicated structure, large-scale equipment, etc.
There has been proposed a method in which a light-transmitting image-bearing member is exposed to a light image corresponding to an original image from a back side to form an electrostatic latent image on a surface of the image-bearing member, and the latent image is developed simultaneously by selectively attracting thereon a magnetic conductive toner in a developer which is supplied by a developing roll composed of a permanent magnet and a sleeve. The developed image is then transferred and fixed onto a recording sheet.
Although such recording method employing a rear side exposure shows a good developability because a magnetic toner having a specific volume resistance of 10.sup.4 -10.sup.12 .OMEGA..multidot.cm (so-called medium electric resistance) is used, the transferring efficiency of the developed image to a recording sheet is low. Namely, the developed image is not completely transferred onto a recording sheet even when a corotron which is a most general transferring means is used, thereby resulting in blurred image. Therefore, an ordinary paper has not been used in a recording method employing a rear side exposure.
After transferring the developed toner image to a recording sheet, a small amount of the toner is likely to remain on the photosensitive surface of an image-bearing member. Thus, a cleaning device is generally provided to remove the residual toner from the image-bearing member. To this end, a space for installing the cleaning device must be provided in the vicinity of the image-bearing member, failing to achieve an intended miniaturization of an electrophotographic recording apparatus.
When a mixture of a toner and a magnetic carrier is used as the developer, the surface of the magnetic carrier is generally coated with a resin material to improve the durability thereof and control the electrostatic charge of the toner. However, such resin coating is not desirable because it causes various drawbacks such as high specific volume resistance of the magnetic carrier, reduction in developability, low cleaning efficiency of the toner remaining on the photosensitive surface of an image-bearing member, etc. To control the specific volume resistance of the magnetic carrier, there has been known a method where the surface of the carrier particle is partially or completely coated with a resin containing an electrically-conductive particles such as carbon black and metal power, or a method where electrically-conductive particles are added to the carrier after the whole or partial surface of the carrier is coated with a resin. However, the former method cannot reduce the specific volume resistance of the carrier sufficiently. Although the increased content of the electrically-conductive particles can reduce the specific volume resistance, the formed coating layer involves various disadvantages such as easy peeling, etc. In the later method, the detached electrically-conductive particles likely contaminate the surface of the image-bearing member because the adhering force of the electrically-conductive particles to the resin layer is low.
For the two-component developer for use in usual electrophotography, it has been proposed to add the electrically-conductive particles onto the surface of resin-coated magnetic carrier to improve the image quality. However, the same problems mentioned above are involved also in this case.